The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These protection devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed. Interruption of the circuit reduces the risk of injury or the potential of property damage from a resulting fire.
Circuit breakers are a preferred type of circuit interrupters because a resetting mechanism allows their reuse. Typical circuit breakers are disclosed in U.S. Pat. Nos. 2,902,560, 3,098,136, 4,616,199 and 4,616,200, assigned to the instant assignee and incorporated herein by reference. These circuit breakers are designed to trip open and interrupt an electric circuit in response to detecting short circuits and overloads. Short circuit protection is provided by an electromagnetic element that trips when sensing high current flow. The elevated current level causes a high magnetic flux field around a yoke to draw a magnetic armature toward the yoke. The magnetically-drawn armature rotates about a pivot and a trip lever is released from its engagement with the armature. The release and movement of the trip lever causes moveable contacts to separate, thereby opening the circuit breaker and interrupting the electrical circuit.
Overload protection is provided by a thermal bimetal element which will bend when heated by the increased current, causing the circuit breaker to trip and interrupt the power. This can occur when too many loads draw power from the same branch circuit at the same time, or when a single load draws more power than the branch circuit is designed to carry. The bimetal is composed of two dissimilar thermostat materials which are laminated or bonded together and which expand at different rates due to temperature increase, thereby causing the bimetal to bend. The yoke and armature described in relation to magnetic tripping are connected to the bimetal, so the yoke and armature are carried with the bimetal as it bends. As described above in connection with magnetic tripping, this causes the armature to release its engagement of the trip lever, thereby causing the movable contacts to open and interrupt the electrical circuit.
Other types of circuit breakers will interrupt an electrical circuit in response to detecting ground faults or arcing faults. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor such as a grounded conductor, a person causing a current path to ground, or an arcing fault to ground. When a ground fault occurs and some current is leaked to ground, a circuit breaker with ground fault detection circuitry detects the difference in current in the line and neutral power conductors. If the fault level exceeds the trip level of the ground fault circuitry, the ground fault circuitry opens the moveable contacts and interrupts the electrical circuit.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor, between two conductors supplying a load, or between a conductor and ground. An arcing fault may be caused by corroded, worn or damaged wiring or insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. When an arcing fault occurs, a circuit breaker with arcing fault detection circuitry detects the rate of change of current in a line conductor. If the rate of change exceeds a predetermined threshold, the circuitry opens the moveable contacts and interrupts the electrical circuit.
There are several problems associated with prior art tripping mechanisms. First, the bimetal associated with a thermal trip must be individually calibrated to achieve precise tripping characteristics in each individual circuit breaker. This calibration procedure is time consuming and expensive. Accordingly, there is a need to provide a circuit breaker without a bimetal in order to eliminate the calibration requirement. Second, the bimetal, yoke and armature tripping mechanism associated with conventional circuit breakers create inflexibility in features and components. Accordingly, there is a need to improve the conventional tripping mechanisms.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.